The term navigation can be broadly defined as the means by which a craft is given guidance to travel from one known location to another. Irrespective of the size, speed or type of craft, or duration of travel, arrival at the desired destination is of paramount importance. Historically, a variety of methods have been invoked in order to accomplish navigation of a moving vehicle. With the advent of satellite position determination systems, such as the NavStar GPS, operated by the U.S. Government, precise position determination within a few meters can be accurately determined and a travel path interpolated by virtue of recording several such data points. Thus, in most instances GPS type systems have become a preferred method of navigating position determination.
The GPS system is comprised of a constellation of earth orbiting spacecraft that continuously transmit dual frequency telemetry that provide timing information relative to the specific spacecraft. A user having a GPS receiver, tunable to either of the two frequencies (L1 or L2) is able to accurately determine his position relative to the earth's surface by virtue of acquiring the signals from four or more spacecraft and determining the range of the receiver from each craft. Well known processing techniques are then utilized in order to provide the user meaningful longitude, latitude and altitude information. One of the two codes utilized, precise code or P-code, has an exceptionally long data sequence modulated at 10.23 MHz. The other telemetry stream is referred to as course/acquisition mode or C/A-code and is a gold code sequence having a chip rate of 1.023 MHz. The gold-code sequence is a well known conventional pseudo-random sequence repeated once every millisecond. Positional accuracy between the two codes varies greatly with the P-code providing the more precise measurement. In order to increase the accuracy of the C/A-code many GPS receivers work in combination with an additional signal emanating from a known location or source, thereby providing accuracy equal to or exceeding P-code. This approach is generally referred to as differential GPS, and is well known in the art.
Initial GPS receivers were quite large and expensive thereby mandating user platforms to relatively large land based or airborne vehicles. As a result of the continued evolution in the electronics industry, the physical size of individual components has shrunk exponentially, while simultaneously exhibiting functional increases of vast proportion. As a direct result of such electronic evolution, the unit price of good quality GPS receivers has continued to decrease such that prices less than five hundred dollars per unit are rather typical. The attainment of relatively low receiver unit pricing affords GPS technology to many applications heretofore considered inappropriate or non-cost beneficial for such applications.
Unmanned vehicles whether self-propelled or projected, represent a new arena of items that could benefit from GPS technology. Whether utilized in a manner to provide control and guidance during the course of flight, or whether the results of a first flight is utilized for adjustments in the trajectory of subsequent items, the inclusion of miniaturized GPS receivers on or in such items provides benefits heretofore not available to such users.
By way of example, large mortar shells directed at medium range targets (30-50 kilometers) determine target distance by virtue of a "registration" shell fired at the target. The registration shell that contains a portion of a GPS receiver and a data translator, which in effect transmits the received GPS data over a wideband signal to a ground user at a remote location. Unfortunately, a wideband analog translator is typically easily detected by surrounding users, contains no additional telemetry capability, and by virtue of size and power requirements is limited to a single frequency. Special ground equipment is required in order to receive the repeated signal from the registration shell, and in general, the system cannot accommodate more than one registration shell in flight at a given time. The accuracy provided the user of such system is generally considered to warrant the increased risk by minimizing the number of attempts necessary to place an object in a given location, and the corresponding time period from beginning to end of the mission. However, improvements in the prior art system which would provide increased stealth capability, accommodate telemetry down-link and the providing of control signals to the shell would be advantageous. Additionally, the minimization of any gear required to support such operations would also be of great benefit.
Accordingly, a need exists for an improved method of navigating and monitoring the results of such navigation of unmanned apparatus such as high-velocity missiles or shell projectiles.